Capture of weapons from armed persons is problematic in its own rite. The problem is that they will frequently use up much of their ammo and may even ruin the weapon while trying to prevent you from capturing anything. They also will almost never be carrying any more than a single combat load per person. This means that you may have to engage in several capture operations in order to accumulate just a basic arsenalís worth of ammunition and guns for spare parts. You also face danger every time that you attempt to capture weapons from anybody. I once encountered a street thug who tried to rob me of a pistol I was carrying. He assumed that his relatively good streetfighting skills, the element of surprise, and his crack emboldened self confidence could carry it through, even though he was unarmed. The coroner never could precisely determine how many times I shot the hoodlum, only that he ended up dead.
Traditional guerrillas have relied heavily on captured weapons, but the crutch has always been outside sponsorship and/ or their ability to buy at least some weapons on the open market. The survival group, however may not be motivated by high ideals and whatnot, but may borrow from some principals of guerrilla warfare. On the other hand, survivalism is about being prepared ahead of time, not stop gap desperate last minute measures. If you were not prepared ahead of time, you did something wrong.
Capturing weapons from hostile persons or organizations is often very difficult. While many "guerrilla" organizations have depended on captured weapons, your likelihood of capturing weapons is often largely dependent on how well armed you are to begin with. Sure, with some good planning and smart tactics you may be able to capture a few heavy weapons from an enemy force, but this is mostly beyond the scope of what survivalism is about. On the other hand, recovering some weapons as booty from attackers may prove to be an extra source of weapons and equipment if it is done right. Essentially, you rob the robber. You may be able to carefully isolate and ambush some hostiles and take what they have. Thus, a military style ambush for the purpose of capturing weapons is essentially just a big violent robbery, and will be treated as such by the authorities in most cases. With regard to occupying or hostile government forces, this may not be a moral or ethical issue. You also may have some degree of assurance that they will not be around to retaliate against you in some way later on. Hostile military or paramilitary organizations will frequently just become more careful in the future and will take care to remove weapons and valuable supplies from the bodies of their comrades when they withdraw from a battle, even if they lose, you may not capture much gear of any value.
A well planned military style ambush against common outlaws can be devastating to the enemy as well as profitable, at least until they get wise as to how to avoid the ambush. This kind of activity alone could almost eliminate crime in areas where the outlaws have practically taken over. In survival scenarios where more people have made the rational decision to be violent, there may be no shortage of outlaws to capture weapons from, but they will also quickly find ways to cope with the danger. As a survivalist, you may have an interest in ensuring that they see the need to just stay away or take their chances at getting messed up and robbed.